Pulmonary hypertension (PH) is a progressive disease with high morbidity and mortality with mutations in BMPR2 a predisposing factor in this disease. A hallmark of the subcellular alterations in this disease, in man and in experimental models, includes enlarged vacuolated endothelial and smooth muscle cells with increased endoplasmic reticulum and Golgi stacks (megalocytosis) in pulmonary arterial lesions. We propose a novel disease-initiating mechanism at the subcellular level - a dysfunction of intracellular trafficking (the Golgi blockade hypothesis). We shall investigate this mechanism in human PH and in experimental models (monocrotaline, hypoxia, BMPR2R899X mutation) using in vivo-derived materials and in experiments in cell culture. Predicted consequences include trapping of vesicle tethers, SNAREs and SNAPs in the Golgi and subcellular mislocalization of N-ethylmaleimide sensitive factor (NSF), and of cargo such as eNOS and vasorelevant growth factor recetors (SNAREing pulmonary hypertension), Distal consequences include enhanced cytokine and growth factor signaling, promitogenic signaling and DNA synthesis, resistance to apoptosis, increased cell size and cell migration and thus reduction of the vascular lumen. In Aim 1 (in vivo studies) we shall link the Golgi blockade hypothesis to the pathophysiology of idiopathic PH in man, in the monocrotaline and hypoxia models of PH in the rat, and in the BMPR2 R899X transgenic mouse. In Aim 2 (cell culture studies) we shall investigate the initiation mechanisms that lead to Golgi blockade and defects in intracellular trafficking in response to MCTP, hypoxia and BMPR2 mutants with a focus on NSF and the proximal mechanistic events involved. In Aim 3 (cell culture studies) we shall investigate some of the distal trafficking consequences of disruption of intracellular trafficking by MCTP, hypoxia and BMPR mutants. PUBLIC HEALTH RELEVANCE. Pulmonary arterial hypertension (PH) is a progressive fatal lung disease in man. We propose an all together novel way of thinking about the cause of this fatal disease. We suggest that the endothelial and smooth muscle cells that line the arterial blood vessels in the lung become bigger and increase in number because of a defect in cellular machinery that handles the movement of proteins within these cells. This leads to a blockage of the blood vessels. The proposed mechanism opens up new ways of thinking about how to treat this disease.